The invention relates to a method for hardening form substances made out of building materials containing binding agents, especially light porous construction materials, in water/steam inside of a pressure chamber of an autoclave.
In the known methods of this kind, the heat necessary for the hardening of the form substances is supplied by means of saturated steam which is fed into the pressure chamber of the autoclave. The heat supplied by means of the steam essentially is limited to the heating-up phase, while the heat supplied by conveying additional steam during the holding phase is limited to the amount that is necessary for maintaining the predetermined upper limit temperature of the saturated steam in view of the insignificant heat loss occurring in the pressure container. The holding phase period depends on the utilized building materials as well as on the time required for the inside of the form substances to reach the desired equalization temperature for hardening. In the autoclaving of, for example, silicate building materials, the equalization temperature is approximately 156.degree. C., and the upper limit temperature of the saturated steam at a saturation pressure of 16 bar is 203.degree. C. When hardening is completed after reaching the equalization temperature, and after the holding phase, there follows a cooling-off phase during which the steam pressure is usually continuously linearly reduced until it reaches the ambient pressure. Subsequently, the form substances are taken out of the pressure chamber and are ready for further use after complete cooling off to the ambient temperature.
When such a hardening method is used, form substances made of heavy building materials usually attain satisfactory hardness values at a water content in the order or magnitude of approximately 3 to 5% of volume. This order or magnitude complies with the equalization value of the water content of the form substances in masonry.
If, however, form substances made of light building materials, especially, for example, porous silicate or porous concrete, undergo such a hardening method, then it can be observed that they leave the pressure container of the autoclave with a water content in the order or magnitude of approximately 20% of volume which not only impairs the hardness of the form substances but also increases significantly their weight and decreases their heat insulating properties. Only after a long drying period, usually in the walled-up state, do form substances made of such light building material attain their equalization water content of approximately 3 to 5% of volume. Consequently, this leads to considerable additional heating costs in order to dry out the masonry until the form substances reach their equalization water content and thereby their full hardness and heat insulation properties. Because the form substances undergo considerable shrinkage during such a drying process, it is to be expected that the buildings will have cracks.
The high water content of form substances made of light building materials, especially porous light building materials, at leaving the autoclave is due to the fact that the form substances are heavily charged with mixing water, and then absorb additional water during the hardening process from the condensation of steam on the surfaces of the form substances that are able to absorb condensation, whereby the surfaces especially of light building materials are already considerably increased. The water contained in the form substances remains therein almost entirely because the heat content in the form substances made out of light building materials is insufficient to bring about a considerable amount of water reduction by evaporation during the cooling-off phase.
It has already been proposed that for the reduction of water content of completely hardened form substances made of light building materials to submit the form substances to vacuum drying before the hardening treatment in the autoclave . This, however, does not lead to satisfactory results because the form substances absorb considerable amounts of condensation water during the hardening process in the saturated steam atmosphere. Furthermore, such a method is very expensive from the point of view of energy consumption and equipment requirements.
An object of the present invention is to provide a simple, effective and energy-efficient method for hardening form substances made of building materials containing binding agents, especially made of porous light building materials, which can be regulated to approximately 3 to 5% of water by volume.
The method according to this invention goes beyond the traditional fundamental process based upon the utilization of water vapor in the saturated steam range for the purpose of a heating medium, and introduces a heating medium which is "dry". In its simplest application, ambient air can be utilized, whereby the existing vapor content resulting from the relative air humidity can be considered harmless. By prior or simultaneously increasing the pressure in the pressure chamber of the autoclave, it is assured that on the inside the pressure dependent evaporation temperature of water is not reached, even though the temperature is increased over and above the required equalization temperature up to a predetermined upper limit, and, thus the detrimental consequences of working with steam can be avoided. In particular, the form substances are no longer exposed to wet vapor during the heating-up phase, during which vapor would condense on the still cooler form substances and would settle as condensation, in addition to the original mixing water from the shaping, with the result that the water content requirements necessary for the chemical hardening process would be far exceeded.
Because the method of the invention does not exceed the evaporation temperature of water, neither in the heating-up phase, nor in the holding phase, the form substances remain unaffected by evaporation processes which could lead to a deterioration of the structure of the materials, especially before and during the hardening process, and in direct relation to the extent specific ranges are exposed or how deeply removed they are from the heating medium. Significant energy saving also go hand-in-hand with a "steam free" autoclaving method. During autoclaving of building materials by conveying steam into the inner chamber of the autoclave, approximately 40 to 43% of the heat loss results from condensation. In addition, there are considerable energy losses in operating a steam boiler with feedwater deaeration and residue losses, whereas the degree of effectiveness of heating gas can be realized much more favorably. All this leads to great advantages in an autoclaving method with a gaseous heating medium which is kept below the evaporation temperature of water.
Thereby the pressure increase in a closed autoclave chamber cannot be left solely to the thermally induced pressure increase because the isochoric characteristic curve derived at normal conditions also lies above the steam pressure curve of water at temperatures above 156.degree. C. On the other hand, additional pressure is needed, which can be attained at an initial autoclave pressure of 9 bars or more, so that after the heatingup to, for example 156.degree. C., which is considerably above the equalization temperature, and a pressure of 13 or more bars, a state of the gaseous heating medium is maintained which lies below the steam pressure curve.
However, the additional required pressure can also be introduced continuously or gradually during the heating-up phase. Thereby, the heating can take place in the autoclave itself, and specifically with a heating apparatus mounted on or inside it. It is, however, also possible to pass hot gas into the pressure chamber of the autoclave. It can be generated as hot gas on the outside and pumped directly into it, or it can be drawn in from the outside, or the gas circulated through the autoclave can be heated up, for example, by an external heat exchanger.
In an additional utilizable solution of the invention, it is also feasible if the heating-up and the hardening take place with the conventional heating medium of water vapor, either in consideration of utilizing an already-existing steam boiler equipment to utilize water vapor fast heat transmission. The high water content resulting especially with porous light building materials is reduced by a hardening method which integrates a drying method, i.e. by conveying heat during a particular phase, whereby the form substances receive the amount of heat which is required for evaporating the water contained in the form substances up to a desired remaining water content level. The consequent increase in temperature resulting from the saturation stage of the steam in the pressure chamber of the autoclave is prevented by blowing off steam, and the related disturbances in the equilibrium between the water and steam present in the pressure chamber of the autoclave caused by blowing off of steam is equalized by the constant evaporation of water out of the form substances. As far as processing engineering and equipment requirements are concerned, this method assures a simple way of producing form substances of the desired low water content at the time of leaving the pressure chamber of the autoclave. The method of the invention is highly accurately controllable and achieves an excellent degree of effectiveness, whereas, at the same time, existing autoclaves can still be utilized and require only relatively minor modifications before being utilizable for executing the method of the present invention.
A suitable autoclave for the execution of the method needs only to be equipped with a heating apparatus for the pressure chamber. In addition, the autoclave is equipped also with a steam pressure regulation valve in a blowing-off conduit, which usually even in the conventional autoclaves is already in place, and which possesses adjustable regulation characteristics. With the aid of such a regulation valve, it is possible during heat conveyance to preset a temperature/pressure process of the steam in that phase during which the evaporation of water out of the form substances occurs due to heating after reaching the upper limit temperature, which assures an evaporation and energy optimizing method of treatment of form substances.
In principle, it is possible with the method of the present invention to keep using the conventional heat supply during the heating-up phase by conveying steam into the pressure chamber of the autoclave, and to only initiate the heat supply after reaching the upper limit temperatures by heating the inside of the pressure chamber. The preferred method, however, is to provide the entire heat supply during the hardening process by means of pressure chamber heating, because it allows for especially precise regulation and can be constructed less expensively. When hardening form substances which are especially susceptible to develop cracks or to sustain surface damage during the heating-up phase due to evaporation of their water content, such manifestations can be counteracted by introducing a small amount of water into the pressure chamber at the beginning of the heating-up phase so that because of the heating effect, the water relatively quickly is converted into saturated steam which counteracts premature evaporation of water from the form substances.
Other features which are considered characteristic of the invention are set forth in the appended claims.
Although the invention is illustrated and described in relationship to specific embodiments, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.